For the very first time I am exploring expression trees. I have a few basic doubts.
So essentially , an expression takes only a lambda expression . Ans then we can Compile() the lambda expression to MSIL code which in turn returns a generic delegate. We can invoke the returned delegate as it is . Is my understanding correct ?
If it is here is what I am trying to achieve: ((10*5)+(9/4))
BinaryExpression b1 = Expression.MakeBinary(ExpressionType.Multiply, Expression.Constant(10), Expression.Constant(5));//(10*5)
BinaryExpression b2 = Expression.MakeBinary(ExpressionType.Divide, Expression.Constant(9), Expression.Constant(4));//(9/4)
BinaryExpression b4 = Expression.MakeBinary(ExpressionType.Add, b1, b2);//((10*5)+(9/4))
So at this point we have made the lambda expression body . Now to turn it to a full lambda expression we need to call
Console.WriteLine(Expression.Lambda<Func<int, int>>(b4).Compile());
I am not getting this part . And this does not work also .
Why this Func<int,int>?
Is it like the inner expressions will take only int as param and the entire expression will return an int?
Obviously this does not work. How the generated lambda looks like ?
I am getting the entire picture? How to make this work?
Expression.Lambda<Func<int, int>>(b4).Compile()
Func<int,int> is a signature for lambdas that take a single int parameter, and return an int. Your lambda has a different signature.
Obviously this does not work.
Your lambda does not take any parameters, so you need Func<int> instead.
How the generated lambda looks like?
Generated lambda is a callable object. If you would like to evaluate the expression that you get back, cast and call it, like this:
var compiledLambda = (Func<int>)Expression.Lambda<Func<int>>(b4).Compile();
Console.WriteLine(compiledLambda());
// ^^
The above prints 52, as expected.
Demo 1.
If you would like to make a Func<int,int>, add a parameter to your expression, e.g. make it (p*5)+(9/4) where p is an int parameter:
ParameterExpression p = Expression.Parameter(typeof(int));
BinaryExpression b1 = Expression.MakeBinary(ExpressionType.Multiply, p, Expression.Constant(5));//(p*5)
BinaryExpression b2 = Expression.MakeBinary(ExpressionType.Divide, Expression.Constant(9), Expression.Constant(4));//(9/4)
BinaryExpression b4 = Expression.MakeBinary(ExpressionType.Add, b1, b2);
var compiledLambda = (Func<int,int>)Expression.Lambda<Func<int,int>>(b4, new[] {p}).Compile();
Console.WriteLine(compiledLambda(10)); // Prints 52
Console.WriteLine(compiledLambda(8)); // Prints 42
Demo 2.
You can create your lambda expression like that:
LambdaExpression lb = Expression.Lambda(b4);
You can then compile this expression to a delegate:
Delegate dlg = lb.Compile();
And cast this delegate to a Func<int>:
Func<int> f = (Func<int>)dlg;
And you can use this as usual:
Console.WriteLine(f()); // 52
The generic way works, too. Why do you use Func<int,int>? Your expression takes no input and returns a single int:
Func<int> f = Expression.Lambda<Func<int>>(b4);
The generic argument leads to a LambdaExpression with a Compile method that returns a Func<int> instead of a Delegate that you'd need to cast again.
Using:
Console.WriteLine(Expression.Lambda<Func<int, int>>(b4).Compile());
you actually print it with Console.WriteLine(object) overload which prints the name of the Type of the argument.
since the line:
Expression.Lambda<Func<int>>(b4).Compile();
only compiles the lambda and provides you the delegate - not its invocation's result.
You need to invoke the compiled lambda and print only the result:
Func<int> result = (Func<int>)Expression.Lambda<Func<int>>(b4).Compile();
Console.WriteLine(result());
Also note that you try to compile the delegate as a Func<int,int> which takes an int argument and provides an int result but your code doesn't require an argument so you need to use Func<int>.
Action<int, string> s = new Action<int, string>(delegate(int a,string b){});
Action<int, string> ss = delegate(int a, string b) { };
Why these both work? Why the constructor Action can accept both function and
delegator?
You're not really making a constructor call in the normal way, even though that's what it looks like.
Instead, this is a delegate-creation-expression as described in the C# 5 specification, section 7.6.10.5:
A delegate-creation-expression is used to create a new instance of a delegate-type.
delegate-creation-expression:
new delegate-type ( expression )
The argument of a delegate creation expression must be a method group, an anonymous function or a value of either the compile time type dynamic or a delegate-type.
It's almost always simpler to use the implicit conversion from an anonymous function (lambda expression or anonymous method) to a compatible delegate type, which is what your second line does.
If you remember back to C# 1, this was how we had to create delegate instances, although using method groups:
Action action = new Action(SomeMethod);
C# 2 introduced anonymous methods and also the implicit conversion from method groups to delegates:
Action action = SomeMethod;
These days, delegate creation expressions are relatively rare because there are more compact ways of doing the same thing.
Note that in some cases - if you're passing the anonymous function or method group as an argument for a parameter of type Delegate, for example - you can just cast instead of using a delegate creation expression:
Foo((Action<int, string>) delegate(int a, string b) {});
In the first line you explicitly create the Action<int, string> delegate to the anonnymous method. In the second - implicitly.
Can you say what is the use of the ()=> and =>? I saw this in a code. I did not get any reference for this.
this.Dispatcher.BeginInvoke(()=>
{
//some thing..
};
=> is the lambda operator in C# and is read as "goes to". A lambda expression is an anonymous function and can be used to create a delegate.
Your example takes no arguments as indicated by the empty parens preceding the lambda operator. A lambda expression with one argument might look like this:
n => n.toString()
That expression would return the string representation of n, when invoked. A lambda expression can have multiple arguments as well, contained in parentheses:
(n, f) => n.toString(f)
A common use would be in a Func<T>:
Func<int, string> getString = n => n.toString();
int num = 7;
string numString = getString(num);
This is, of course, a silly example, but hopefully helps to illustrate its use.
This notation is that of a lambda expression which takes no argument. If the lambda expression made use of arguments they would be declared in the empty set of parenthesis as in say...
this.Dispatcher.BeginInvoke((x, y) => { do some' with x and/or y }, 12, somevar);
In a nutshell, lambda expressions allows creating "nameless" functions, right where they are needed.
In the example of the question, the BeginInvoke() method requires its first parameter to be a delegate (a "pointer to a method"), which is exactly what this lambda expression provides.
It's a lambda expression that has no parameters.
Check out this page http://codebetter.com/karlseguin/2008/11/27/back-to-basics-delegates-anonymous-methods-and-lambda-expressions/
If you don’t have any parameters, like in our example, you use empty
paranthesis:
() => {…}
I get asked this question a lot and I thought I'd solicit some input on how to best describe the difference.
They are actually two very different things. "Delegate" is actually the name for a variable that holds a reference to a method or a lambda, and a lambda is a method without a permanent name.
Lambdas are very much like other methods, except for a couple subtle differences.
A normal method is defined in a "statement" and tied to a permanent name, whereas a lambda is defined "on the fly" in an "expression" and has no permanent name.
Some lambdas can be used with .NET expression trees, whereas methods cannot.
A delegate is defined like this:
delegate Int32 BinaryIntOp(Int32 x, Int32 y);
A variable of type BinaryIntOp can have either a method or a labmda assigned to it, as long as the signature is the same: two Int32 arguments, and an Int32 return.
A lambda might be defined like this:
BinaryIntOp sumOfSquares = (a, b) => a*a + b*b;
Another thing to note is that although the generic Func and Action types are often considered "lambda types", they are just like any other delegates. The nice thing about them is that they essentially define a name for any type of delegate you might need (up to 4 parameters, though you can certainly add more of your own). So if you are using a wide variety of delegate types, but none more than once, you can avoid cluttering your code with delegate declarations by using Func and Action.
Here is an illustration of how Func and Action are "not just for lambdas":
Int32 DiffOfSquares(Int32 x, Int32 y)
{
return x*x - y*y;
}
Func<Int32, Int32, Int32> funcPtr = DiffOfSquares;
Another useful thing to know is that delegate types (not methods themselves) with the same signature but different names will not be implicitly casted to each other. This includes the Func and Action delegates. However if the signature is identical, you can explicitly cast between them.
Going the extra mile.... In C# functions are flexible, with the use of lambdas and delegates. But C# does not have "first-class functions". You can use a function's name assigned to a delegate variable to essentially create an object representing that function. But it's really a compiler trick. If you start a statement by writing the function name followed by a dot (i.e. try to do member access on the function itself) you'll find there are no members there to reference. Not even the ones from Object. This prevents the programmer from doing useful (and potentially dangerous of course) things such as adding extension methods that can be called on any function. The best you can do is extend the Delegate class itself, which is surely also useful, but not quite as much.
Update: Also see Karg's answer illustrating the difference between anonymous delegates vs. methods & lambdas.
Update 2: James Hart makes an important, though very technical, note that lambdas and delegates are not .NET entities (i.e. the CLR has no concept of a delegate or lambda), but rather they are framework and language constructs.
The question is a little ambiguous, which explains the wide disparity in answers you're getting.
You actually asked what the difference is between lambdas and delegates in the .NET framework; that might be one of a number of things. Are you asking:
What is the difference between lambda expressions and anonymous delegates in the C# (or VB.NET) language?
What is the difference between System.Linq.Expressions.LambdaExpression objects and System.Delegate objects in .NET 3.5?
Or something somewhere between or around those extremes?
Some people seem to be trying to give you the answer to the question 'what is the difference between C# Lambda expressions and .NET System.Delegate?', which doesn't make a whole lot of sense.
The .NET framework does not in itself understand the concepts of anonymous delegates, lambda expressions, or closures - those are all things defined by language specifications. Think about how the C# compiler translates the definition of an anonymous method into a method on a generated class with member variables to hold closure state; to .NET, there's nothing anonymous about the delegate; it's just anonymous to the C# programmer writing it. That's equally true of a lambda expression assigned to a delegate type.
What .NET DOES understand is the idea of a delegate - a type that describes a method signature, instances of which represent either bound calls to specific methods on specific objects, or unbound calls to a particular method on a particular type that can be invoked against any object of that type, where said method adheres to the said signature. Such types all inherit from System.Delegate.
.NET 3.5 also introduces the System.Linq.Expressions namespace, which contains classes for describing code expressions - and which can also therefore represent bound or unbound calls to methods on particular types or objects. LambdaExpression instances can then be compiled into actual delegates (whereby a dynamic method based on the structure of the expression is codegenned, and a delegate pointer to it is returned).
In C# you can produce instances of System.Expressions.Expression types by assigning a lambda expression to a variable of said type, which will produce the appropriate code to construct the expression at runtime.
Of course, if you were asking what the difference is between lambda expressions and anonymous methods in C#, after all, then all this is pretty much irelevant, and in that case the primary difference is brevity, which leans towards anonymous delegates when you don't care about parameters and don't plan on returning a value, and towards lambdas when you want type inferenced parameters and return types.
And lambda expressions support expression generation.
One difference is that an anonymous delegate can omit parameters while a lambda must match the exact signature. Given:
public delegate string TestDelegate(int i);
public void Test(TestDelegate d)
{}
you can call it in the following four ways (note that the second line has an anonymous delegate that does not have any parameters):
Test(delegate(int i) { return String.Empty; });
Test(delegate { return String.Empty; });
Test(i => String.Empty);
Test(D);
private string D(int i)
{
return String.Empty;
}
You cannot pass in a lambda expression that has no parameters or a method that has no parameters. These are not allowed:
Test(() => String.Empty); //Not allowed, lambda must match signature
Test(D2); //Not allowed, method must match signature
private string D2()
{
return String.Empty;
}
Delegates are equivalent to function pointers/method pointers/callbacks (take your pick), and lambdas are pretty much simplified anonymous functions. At least that's what I tell people.
A delegate is a function signature; something like
delegate string MyDelegate(int param1);
The delegate does not implement a body.
The lambda is a function call that matches the signature of the delegate. For the above delegate, you might use any of;
(int i) => i.ToString();
(int i) => "ignored i";
(int i) => "Step " + i.ToString() + " of 10";
The Delegate type is badly named, though; creating an object of type Delegate actually creates a variable which can hold functions -- be they lambdas, static methods, or class methods.
I don't have a ton of experience with this, but the way I would describe it is that a delegate is a wrapper around any function, whereas a lambda expression is itself an anonymous function.
A delegate is always just basically a function pointer. A lambda can turn into a delegate, but it can also turn into a LINQ expression tree. For instance,
Func<int, int> f = x => x + 1;
Expression<Func<int, int>> exprTree = x => x + 1;
The first line produces a delegate, while the second produces an expression tree.
Short version:
A delegate is a type that represents references to methods. C# lambda expression is a syntax to create delegates or expression trees.
Kinda long version:
Delegate is not "the name for a variable" as it's said in the accepted answer.
A delegate is a type (literally a type, if you inspect IL, it's a class) that represents references to methods (learn.microsoft.com).
This type could be initiated to associate its instance with any method with a compatible signature and return type.
namespace System
{
// define a type
public delegate TResult Func<in T, out TResult>(T arg);
}
// method with the compatible signature
public static bool IsPositive(int int32)
{
return int32 > 0;
}
// initiated and associate
Func<int, bool> isPositive = new Func<int, bool>(IsPositive);
C# 2.0 introduced a syntactic sugar, anonymous method, enabling methods to be defined inline.
Func<int, bool> isPositive = delegate(int int32)
{
return int32 > 0;
};
In C# 3.0+, the above anonymous method’s inline definition can be further simplified with lambda expression
Func<int, bool> isPositive = (int int32) =>
{
return int32 > 0;
};
C# lambda expression is a syntax to create delegates or expression trees. I believe expression trees are not the topic of this question (Jamie King about expression trees).
More could be found here.
lambdas are simply syntactic sugar on a delegate. The compiler ends up converting lambdas into delegates.
These are the same, I believe:
Delegate delegate = x => "hi!";
Delegate delegate = delegate(object x) { return "hi";};
A delegate is a reference to a method with a particular parameter list and return type. It may or may not include an object.
A lambda-expression is a form of anonymous function.
A delegate is a Queue of function pointers, invoking a delegate may invoke multiple methods. A lambda is essentially an anonymous method declaration which may be interpreted by the compiler differently, depending on what context it is used as.
You can get a delegate that points to the lambda expression as a method by casting it into a delegate, or if passing it in as a parameter to a method that expects a specific delegate type the compiler will cast it for you. Using it inside of a LINQ statement, the lambda will be translated by the compiler into an expression tree instead of simply a delegate.
The difference really is that a lambda is a terse way to define a method inside of another expression, while a delegate is an actual object type.
It is pretty clear the question was meant to be "what's the difference between lambdas and anonymous delegates?" Out of all the answers here only one person got it right - the main difference is that lambdas can be used to create expression trees as well as delegates.
You can read more on MSDN: http://msdn.microsoft.com/en-us/library/bb397687.aspx
Delegates are really just structural typing for functions. You could do the same thing with nominal typing and implementing an anonymous class that implements an interface or abstract class, but that ends up being a lot of code when only one function is needed.
Lambda comes from the idea of lambda calculus of Alonzo Church in the 1930s. It is an anonymous way of creating functions. They become especially useful for composing functions
So while some might say lambda is syntactic sugar for delegates, I would says delegates are a bridge for easing people into lambdas in c#.
Some basic here.
"Delegate" is actually the name for a variable that holds a reference to a method or a lambda
This is a anonymous method -
(string testString) => { Console.WriteLine(testString); };
As anonymous method do not have any name we need a delegate in which we can assign both of these method or expression. For Ex.
delegate void PrintTestString(string testString); // declare a delegate
PrintTestString print = (string testString) => { Console.WriteLine(testString); };
print();
Same with the lambda expression. Usually we need delegate to use them
s => s.Age > someValue && s.Age < someValue // will return true/false
We can use a func delegate to use this expression.
Func< Student,bool> checkStudentAge = s => s.Age > someValue && s.Age < someValue ;
bool result = checkStudentAge ( Student Object);
Lambdas are simplified versions of delegates. They have some of the the properties of a closure like anonymous delegates, but also allow you to use implied typing. A lambda like this:
something.Sort((x, y) => return x.CompareTo(y));
is a lot more concise than what you can do with a delegate:
something.Sort(sortMethod);
...
private int sortMethod(SomeType one, SomeType two)
{
one.CompareTo(two)
}
Heres an example I put up awhile on my lame blog. Say you wanted to update a label from a worker thread. I've got 4 examples of how to update that label from 1 to 50 using delegates, anon delegates and 2 types of lambdas.
private void button2_Click(object sender, EventArgs e)
{
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += new DoWorkEventHandler(worker_DoWork);
worker.RunWorkerAsync();
}
private delegate void UpdateProgDelegate(int count);
private void UpdateText(int count)
{
if (this.lblTest.InvokeRequired)
{
UpdateProgDelegate updateCallBack = new UpdateProgDelegate(UpdateText);
this.Invoke(updateCallBack, new object[] { count });
}
else
{
lblTest.Text = count.ToString();
}
}
void worker_DoWork(object sender, DoWorkEventArgs e)
{
/* Old Skool delegate usage. See above for delegate and method definitions */
for (int i = 0; i < 50; i++)
{
UpdateText(i);
Thread.Sleep(50);
}
// Anonymous Method
for (int i = 0; i < 50; i++)
{
lblTest.Invoke((MethodInvoker)(delegate()
{
lblTest.Text = i.ToString();
}));
Thread.Sleep(50);
}
/* Lambda using the new Func delegate. This lets us take in an int and
* return a string. The last parameter is the return type. so
* So Func<int, string, double> would take in an int and a string
* and return a double. count is our int parameter.*/
Func<int, string> UpdateProgress = (count) => lblTest.Text = count.ToString();
for (int i = 0; i < 50; i++)
{
lblTest.Invoke(UpdateProgress, i);
Thread.Sleep(50);
}
/* Finally we have a totally inline Lambda using the Action delegate
* Action is more or less the same as Func but it returns void. We could
* use it with parameters if we wanted to like this:
* Action<string> UpdateProgress = (count) => lblT…*/
for (int i = 0; i < 50; i++)
{
lblTest.Invoke((Action)(() => lblTest.Text = i.ToString()));
Thread.Sleep(50);
}
}
I assume that your question concerns c# and not .NET, because of the ambiguity of your question, as .NET does not get alone - that is, without c# - comprehension of delegates and lambda expressions.
A (normal, in opposition to so called generic delegates, cf later) delegate should be seen as a kind of c++ typedef of a function pointer type, for instance in c++ :
R (*thefunctionpointer) ( T ) ;
typedef's the type thefunctionpointer which is the type of pointers to a function taking an object of type T and returning an object of type R. You would use it like this :
thefunctionpointer = &thefunction ;
R r = (*thefunctionpointer) ( t ) ; // where t is of type T
where thefunction would be a function taking a T and returning an R.
In c# you would go for
delegate R thedelegate( T t ) ; // and yes, here the identifier t is needed
and you would use it like this :
thedelegate thedel = thefunction ;
R r = thedel ( t ) ; // where t is of type T
where thefunction would be a function taking a T and returning an R. This is for delegates, so called normal delegates.
Now, you also have generic delegates in c#, which are delegates that are generic, i.e. that are "templated" so to speak, using thereby a c++ expression. They are defined like this :
public delegate TResult Func<in T, out TResult>(T arg);
And you can used them like this :
Func<double, double> thefunctor = thefunction2; // call it a functor because it is
// really as a functor that you should
// "see" it
double y = thefunctor(2.0);
where thefunction2 is a function taking as argument and returning a double.
Now imagine that instead of thefunction2 I would like to use a "function" that is nowhere defined for now, by a statement, and that I will never use later. Then c# allows us to use the expression of this function. By expression I mean the "mathematical" (or functional, to stick to programs) expression of it, for instance : to a double x I will associate the double x*x. In maths you write this using the "\mapsto" latex symbol. In c# the functional notation has been borrowed : =>. For instance :
Func<double, double> thefunctor = ( (double x) => x * x ); // outer brackets are not
// mandatory
(double x) => x * x is an expression. It is not a type, whereas delegates (generic or not) are.
Morality ? At end, what is a delegate (resp. generic delegate), if not a function pointer type (resp. wrapped+smart+generic function pointer type), huh ? Something else ! See this and that.
Well, the really oversimplified version is that a lambda is just shorthand for an anonymous function. A delegate can do a lot more than just anonymous functions: things like events, asynchronous calls, and multiple method chains.